A chemo-mechanical coupling model of deviated borehole stability in hard brittle shale

A chemo-mechanical coupling model of borehole stability in hard brittle shale considering structure characteristics and targeted hydration was established, the influencing factors of the distribution of collapse pressure were analyzed based on the model, and a field case analysis was conducted. Based on the physicochemical properties of hard brittle shale, a drilling fluid activity window was proposed for calculating collapse pressure by establishing the relationships of drilling fluid activity vs. swelling ratio of rock and rock activity vs. moisture content to determine critical swelling ratio of rock and reasonable moisture content. The results show that, when fixing the dip angle of weak plane, the collapse pressure appears a quarter symmetric distribution with the change in tendency, there is no azimuth angle who has a monotonic increasing or decreasing collapse pressure, and dangerous sections and safe sections exist alternately; compared with cohesion of weak plane, collapse pressure is more sensitive to internal friction angle. Field case shows that, accurate prediction of collapse pressure distribution can be obtained by the chemo-mechanical coupling model, in which borehole stability can be ensured and the density of drilling fluid can be decreased as long as the drilling fluid activity is controlled in the window.